This invention relates generally to machine tools having hollow spindles and hollow chucks mounted thereon. More particularly, the invention relates to a fluid-actuated type hollow power chuck in which a fluid-actuated cylinder for driving the jaws of the chuck is provided integrally with the chuck body in order to utilize with maximum effectiveness the through bore of the main spindle of a machine tool. Chucks of this character are generally called "front chucks" and are often used for machining pipes for oil drilling work and electrode rods for electric furnaces. These chucks are suitable for machining rod and tube materials of relatively large diameters.
Recently, in the field of machine tools, the use of numerical-control machine tools has become widespread. In many cases, however, in order to elevate the work reliability in such numerical-control machine tools, verification of the gripped and released states of the workpiece by electrically detecting the movement of the jaws of the chuck is required.
For this purpose, in an ordinary type of fluid-actuated chuck in which the chuck and a fluid cylinder for actuating the chuck jaws are separate, the piston rod of the piston operating in the cylinder projects rearwardly of the cylinder, which is mounted on the rear end of the machine spindle, and the position of the extremity of this piston rod is detected at a point to the rear of the cylinder by limit switches or proximity switches thereby to detect the movements of the chuck jaws.
In contrast, in a fluid-actuated hollow chuck in which the chuck and the jaw-actuating cylinder are of integral construction, from the viewpoint of the fundamental functional object of the chuck, the disposing of a tube or rod for transmitting the movements of the hollow piston to a point rearward of the machine spindle within the through bore of the spindle is not permissible. For this reason, detecting of the movements of the jaws at a point to the rear of the spindle as in the aforedescribed ordinary type chuck is not possible, whereby it has been unavoidably necessary to carry out this detecting at a point forward of the spindle.
In a known fluid-actuated hollow chuck of this kind, as shown in FIG. 5 of Japanese Patent Publication No. 34354/1978 (corresponding to U.S. Pat. No. 3,918,352), a rear cover secured to the rear face of the chuck body on the inner side of which a cylinder chamber is formed is mounted on the machine spindle, and a guiding bushing is fitted in the chuck body and a central hole of the rear cover and fixed to the chuck body, a hollow chuck piston being slidably fitted around the outer surface of this guiding bushing.
In this chuck, it may appear to be possible to provide on the rear side of the hollow piston a piston sleeve projecting rearward of the rear cover and to detect the position of this piston sleeve by means of switches. However, because of the construction of this chuck as described above, this piston sleeve must be disposed on the inner side of the attachment of the rear cover and the spindle, whereby it is practically impossible to detect movements of this piston sleeve. Another method of detection which may seem to be feasible is to embeddedly secure a plurality of pins to the hollow piston, to lead the movements of these pins outside of the chuck body, and to detect these movements. However, a system for practicing this method would be complicated and difficult to reduce to practice. Thus, in the present state of the art, a practical detection system capable of electrically detecting the movements of the chuck jaws has not yet been devised.
Furthermore, in a known fluid-actuated hollow chuck of the above description, the chuck body is secured to the machine spindle by way of a rear cover of relatively small thickness dimension, whereby the rigidity of the chuck cannot be increased in spite of the increase in weight of the entire chuck, and it has heretofore been difficult to use chucks of this character in heavy machining.